There is not really anything surprising about that. People can have injuries in the moment and still move around or behave strangely, then die soon thereafter.
She could have had a head injury that resulted in bleeding in the brain-death doesn’t necessarily occur instantly in that scenario. She could have had lung damage in the fire that resulted in lack of oxygen to her brain, resulting in eventual brain death. Hypoxia can cause people to be confused/restless/agitated.
There are plenty of legit reasons why someone could be conscious and appear “upset” or agitated at the scene of an accident and later succumb to their injuries. Happens all the time.
I once worked at the trauma ICU of a large county hospital. Car accidents and what they do to the human body terrified me. Saw a lot of brain dead patients there. I eventually had to get out-I couldn’t emotionally distance myself the way most nurses and doctors could.